A method of obtaining a powder of a valve metal

 

(57) Abstract:

The invention relates to powder metallurgy and can be used to obtain high-purity powders of tantalum and niobium with a large specific surface for the production of capacitors. The method includes the introduction of a salt of a valve metal and the halide of an alkali metal, and supplements in a reactor made of Nickel-containing material, melting salts in the presence of additives and the restoration of a valve metal, an alkaline metal in an atmosphere of inert gas under stirring of the melt, at the same time as supplements use of a valve metal, which is injected in powder form in the amount of 0.3 to 3.0% by weight of the salt of a valve metal in the melt and in the interaction of the additive with the melt and the material of the reactor to form intermetallic protective coating on the inner surface of the reactor. As the valve metal is used tantalum or niobium, and the alkali metal halide is sodium chloride or potassium chloride. The technical result consists in increasing the quality of the powder of a valve metal. Contamination of the metal impurities, in particular Nickel, present in mA who I 1.5-2.0 times the oxygen content in the powder. 1 S. and 2 C. p. F.-ly, 1 table.

The invention relates to ferrous metallurgy and can be used to obtain high-purity powders of tantalum and niobium with a large value of specific surface area.

Among the numerous applications of valve metals has received widespread use them in the form of powders in the production of texturized capacitors, which are characterized by parameters such as the capacitance value, the breakdown voltage and leakage current. These parameters depend on the quality of the powder of a valve metal and process of manufacturing the anode of the capacitor. The anodes are usually made by pressing powder of a valve metal, sintering the preform in the furnace followed by the application of anodic oxide dielectric film polarization in solutions. The quality of the powder is determined by the magnitude of the surface and chemical purity, as the capacity of the anode of the capacitor at a given voltage proportional to the magnitude of its surface, and metallic and non-metallic impurities cause degradation of the dielectric film, which leads to increased leakage current. In the production of capacitors typically use high-purity powders of valve metals with a large amount of surface is

A method of obtaining powder of a valve metal (see U.S. Pat. GERMANY N 2909786, IPC3B 22 F 9/24, 1984), which includes the introduction in the reactor salt of a valve metal composition R2MX7where R is alkali metal, M is tantalum or niobium, X is a halogen from the group of F, Cl, Br, as well as sodium chloride or potassium, melting salts and restoration of a valve metal, an alkaline metal in an atmosphere of helium at a temperature of 760-1000oC with continuous stirring of the melt. The reactor is made from an alloy based on Nickel.

The disadvantage of this method is that due to the active interaction of the melt with the material of the reactor in powder of a valve metal goes a considerable amount of impurities, giving rise to leakage currents. The use of high (1600oC) sintering temperature in the manufacture of capacitors of such powders reduces the leakage current to an acceptable level. However, this significantly reduces the value of the surface of the anode and, therefore, reduced specific capacity that requires greater consumption of tantalum powder per unit of product.

Also known is a method of obtaining powder of a valve metal (see U.S. Pat. USA N 5234491, IPC5B 22 F 9/18, 1993), including the introduction of salts of Nickel-based or iron, melting salts and additives, heating the melt to a temperature of 800-900oC and restoration of a valve metal, an alkaline metal in an argon atmosphere with stirring of the melt. As the halide of the alkali metals are chlorides or fluorides of sodium or potassium, and as an active additive is an alkaline or alkaline earth metals. The basic requirement, which must satisfy the additive is its higher thermodynamic potential and chemical activity in comparison with the material of the reactor and the stirrer. The effect of the active additive is advanced relative to the melt interaction with residual moisture and oxygen present in the atmosphere of the reactor. The active additive is introduced in the form of compact alkaline or alkaline-earth metal, which is a metal rod.

The disadvantage of this method is that the amount of impurities in the melt of the material of the reactor, remains significant. In addition, supplements, interacting with moisture and oxygen present in the reactor, to form oxides, which fall into the melt and, as centers of crystallization when recovering tantalum from its salts, can be the aimed at solving the problems of improving the quality of the powder of a valve metal by reducing contamination of the metal impurities, present in the material of the reactor, providing a large amount of the surface of the powder.

The problem is solved in that in the method of producing powder of a valve metal, including the introduction of a salt of a valve metal and the halide of an alkali metal, and supplements in a reactor made of Nickel-containing material, melting salts in the presence of additives and the restoration of a valve metal, an alkaline metal in an atmosphere of inert gas under stirring of the melt, according to the invention as an active additives use of a valve metal, which is injected in powder form in the amount of 0.3 to 3.0% by weight of the salt of a valve metal in the melt, and the interaction of the additive with the melt and the material of the reactor to form intermetallic protective coating on the inner surface of the reactor.

The problem is solved and the fact that as the valve metal is used tantalum or niobium.

The task contributes to the fact that as alkali metal halide using sodium chloride and/or potassium chloride.

Use directly a valve metal as supplements do not HV is a high metal while restoring its salt with an alkaline metal.

The use of supplements in powder form compared to compact the material gives multiple increase of the surface additives in contact with the melt, which provides a quick melt is saturated with ions of a valve metal to a lower valence and thereby increases the rate of formation of the intermetallic protective coating.

The introduction of active additive in the amount of 0.3 to 3.0% by weight of the salt of a valve metal in the melt promotes the formation of a continuous protective coating on the inner surface of the reactor in contact with the melt. With the introduction of active additives in quantities of less than 0.3 wt.% not form a continuous layer of protective coating and as a consequence may be contamination of the powder by the impurities of the material of the reactor. The introduction of the additive in excess of 3.0 wt.%, does not affect the rate of formation of the protective coating and leads to additional consumption of supplements.

Forming on the inner surface of the reactor protective coating of intermetallic compounds formed by the interaction of powdered valve metal melt and Nickel-containing material of the reactor, ven is and reducing contamination of the metal impurities, present in the material of the reactor.

The use of tantalum or niobium as a valve metal allows you to receive according to the proposed method of anodic oxide films with good dielectric characteristics, which provides greater reliability, high specific charge and small leakage currents are produced on the basis of capacitors.

The use of sodium chloride and/or potassium chloride as the halide of the alkali metal reduces the melting temperature and melt viscosity and provides the ability to control the heat balance in the reaction of repair of a valve metal, thereby obtaining a high quality powder with a large specific surface. Along with sodium chloride and/or potassium chloride as the halide of the alkali metal can be used as a fluoride of sodium and/or potassium fluoride, and a mixture of chlorides and fluorides of sodium and potassium. However, sodium fluoride and potassium fluoride have a higher melting point than sodium chloride or potassium chloride, and, in addition, the potassium fluoride is hygroscopic. Therefore, to reduce the melting temperature of the initial melt more preferred may be explained by the following examples.

Example 1. In a reactor made of Nickel in the form of a cylinder with a height of 400 mm with an inner diameter of 260 mm, a load of 10.0 kg salt K2TaF7, 7.5 kg of salt NaCl and 30 g of tantalum metal powder. Then the reactor was placed in a stainless steel container with a water-cooled cover, which vacuum up to a pressure of 10-2Torr, filled with argon, heated to 800oC and kept at this temperature for 1 hour to melt the mixture of salts. After that, the temperature in the container is reduced to 700oC and under continuous stirring of the melt within 1.3 hours served in the reactor molten sodium in the amount of 3.1 kg In the process of reduction reaction of tantalum gradually increase the temperature of the melt to 820oC. After holding at this temperature for 0.5 hours the reactor is cooled to room temperature. The resulting reaction mass is then removed from the reactor, crushed and washed in water. Washed from salts of tantalum powder is treated consistently in a 10% HCl solution and 1% solution of HF, which take, respectively, in the amount of 1.0 and 0.5 l/kg powder, washed thoroughly with distilled water and dried.

After washing the reactor from salts on its internal on top of the basic analysis, it consists of intermetallic Ni3Ta and Ni2Ta. Studies conducted using optical and electronic microscopy and electron microprobe analyzer Cameca" indicate the continuity and uniformity of the coating.

Characteristics of the tantalum powder obtained in example 1, and the powders obtained in examples 2, 3 and example 4 of the prototype shown in the table.

Example 2. In a reactor made of Monel ( Ni - 73%, Cu - 23%, Fe - 3%, Mn - 1%), in the form of a cylinder with a height of 260 mm with an inner diameter of 150 mm, load 3.0 kg salt2TaF7, 2.8 kg of salt KCl, and 40 g of tantalum metal powder. Then the reactor was placed in a stainless steel container with a water-cooled cover, which vacuum up to a pressure of 10-2Torr, filled with argon, heated to 800oC and kept at this temperature for 0.6 hours to melt the mixture of salts. After that, the temperature in the container is reduced to 720oC and under continuous stirring of the melt within 0.6 hours served in the reactor molten sodium in the number 0,92 kg In the process of reduction reaction of tantalum gradually increase the temperature of the melt up to 830oC. After holding at this temperature for 0.5 hours the reactor is cooled to comnor salts on the inner surfaces of the were in contact with the melt, fixed coating thickness of 5 μm. According to x-ray phase analysis it consists of intermetallic Ni3Ta and Ni2Ta. Studies conducted using optical and electronic microscopy and electron microprobe analyzer Cameca" indicate the continuity and uniformity of the coating.

Example 3. In a reactor made of Nickel in the form of a cylinder with a height of 200 mm, an inner diameter of 140 mm, a load of salt: 1.5 kg K2NbF7, 0.85 kg of NaCl, 0.65 kg of KCl, and 45 g of niobium powder. Then the reactor was placed in a stainless steel container with a water-cooled cover, which vacuum up to a pressure of 10-2Torr, filled with argon, heated to 800oC and kept at this temperature for 0.5 hours to melt a mixture of salts. After that, the temperature in the container is reduced to 730oC and under continuous stirring of the melt in the course of 0.5 hour fed into the reactor molten sodium in an amount of 0.6 kg In the process of reduction reaction of tantalum gradually increase the temperature of the melt to 840oC. After holding at this temperature for 0.2 hours the reactor is cooled to room temperature. The process odmevi, were in contact with the melt, fixed coating thickness of 6 μm. According to x-ray phase analysis it consists of intermetallics NiNb and Ni3Nb. Studies conducted using optical and electronic microscopy and electron microprobe analyzer Cameca" indicate the continuity and uniformity of the coating.

From the above examples show that the proposed method can improve the quality of the powder of a valve metal. Contamination of the metal impurities, in particular Nickel, present in the material of the reactor is reduced to 3-5 times with increasing surface area of the powder by 10-40%. Compared with the prototype of the proposed method produces a powder with low (1.5-2.0 times) oxygen content.

1. A method of obtaining a powder of a valve metal, including the introduction of a salt of a valve metal and the halide of an alkali metal, and supplements in a reactor made of Nickel-containing material, melting salts in the presence of additives and the restoration of a valve metal, an alkaline metal in an atmosphere of inert gas under stirring of the melt, characterized in that as an active additives isolatable in the melt and in the interaction of the additive with the melt and the material of the reactor to form intermetallic protective coating on the inner surface of the reactor.

2. The method according to p. 1, characterized in that as the valve metal is used tantalum or niobium.

3. The method according to p. 1 or 2, characterized in that the alkali metal halide using sodium chloride or potassium chloride.

 

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SUBSTANCE: method is realized at using as corrosion protection means layer of halide of alkali metal formed on inner surface of vessel before creating in reaction vessel atmosphere of inert gas. Charge contains valve metal compound and halide of alkali metal. It is loaded into reaction vessel and restricted by protection layer of halide of alkali metal having melting temperature higher than that of charge by 50 - 400°C. Before loading charge, valve metal compound and alkali metal halide may be mixed one with other. Mass of protection layer of alkali metal halide Ml and charge mass Mc are selected in such a way that that to satisfy relation Ml = k Mc where k - empiric coefficient equal to 0.05 - 0.5. Gas atmosphere of reaction vessel contains argon, helium or their mixture. Fluorotantalate and(or) oxyfluorotantalate or fluoroniobate and(or) oxyfluoroniobate of potassium is used as valve metal compound. Sodium, potassium or their mixture is used as alkali metal. Chloride and(or) fluoride is used as alkali metal halide. Valve metal compound and alkali metal halide may contain alloying additives of phosphorus, sulfur, nitrogen at content of each additive in range 0.005 - 0.1% and 0.005 - 0.2% of mass valve metal compound respectively. Invention lowers by 1.3 - 2 times contamination of powder with metallic impurities penetrating from vessel material. Value of specific surface of powder is increased by 1.2 - 1.8 times, its charge is increased by 10 - 30 %, leakage current are reduced by 1.2 - 1.5 times.

EFFECT: improved quality of valve metal powder, enhanced efficiency of process due to using heat separated at process of reducing valve metal for melting protection layer.

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EFFECT: enhanced efficiency of method.

1 tbl, 3 ex

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